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This invention relates to a porcelain bushing for transformers and, more particularly, relates to means for improving the ability of such a bushing to withstand lightning impulse voltages rated at 45 kV and 1.2/50 xcexcs form without external flashover or puncture.
A bushing is a device that enables one or several conductors to pass through a partition such as a wall or a transformer tank, and insulates the conductor from it. Bushings commonly comprise an insulating bracket, a conductor, and provision for mounting the bracket to the transformer to insulate the conductor from the transformer surface. The bushings are used to hold the electrical conductors and transfer current from outside of the transformer to coils on the inside of the transformer without shorting out on the transformer surface.
Bushings are rated on how well they insulate the transformer wall from current flowing through the conductor. One factor that must be considered in the rating of bushing terminals is the presence of contaminants the bushing. Contaminants on the bushing provide a conductive path, which can cause electricity to leak out onto the transformer wall and cause electrical arcing or striking. Arcing occurs when the difference in potential between the transformer wall and the conductor becomes sufficiently large. The air ionizes between the transformer wall and the conductor, and this creates a path of relatively low resistance through which current can flow. The resulting blast of electricity can cause a short circuit to occur and severely damage the transformer.
Another factor used to rate bushings is electrical xe2x80x9ccreepagexe2x80x9d across the insulating bracket. Creepage is the electrical leakage on a solid dielectric surface. Creepage distance is the shortest distance on a dielectric surface between two conductive elements. The current will essentially track or crawl across the insulating bracket onto the transformer wall. The onset of creepage can produce similar effects to contamination in that a short circuit to the transformer wall can occur and cause damage to the transformer itself.
One solution utilized to minimize these problems is the use of a material with a high insulating property such as porcelain to create the bushing. However, porcelain is not durable and is easily broken. A broken or damaged bushing requires costly replacement and down time of the transformer. Utilization of Ex-Mount bushing a t a higher voltage will result in higher incidents of arcing, striking, or creepage across the insulating bracket. The following are patents related to transformer bushings.
Shirakawa, et al. in the U.S. Pat. No. 6,466,425 provided an arrester excellent in a limit voltage characteristics which can withstand a contamination phenomenon of a porcelain bushing surface, and a surge absorption. Metal plates each having a thickness greater than a zinc oxide element portion does using high withstand voltage zinc oxide elements are arranged to suppress a rise in temperature of the zinc oxide elements. By employing the above means, it is possible to realize an arrester capable of suppressing a rise in temperature of the zinc oxide elements and which is low in a limit voltage and has higher performance.
Smith, et al. in the U.S. Pat. No. 6,140,573 disclosed an improved hollow core composite bushing for use in, for example, a dead tank circuit breaker, is smaller and lighter and offers other advantages over previous designs. The hollow core bushing includes therein a fiber reinforced plastic (FRP) elongated tube that operates to increase the dielectric constant between a conductor and a transition shield. By placing the transition shield external to the hollow core bushing and providing the FRP tube between the conductor and transition shield, the dielectric constant between the shield and conductor is increased thus allowing the inner diameter of the shield and outer diameter of the bushing to be decreased. In addition, the transition shield may be an integral part of the flange that is mounted to the bushing current transformer support column or interrupter enclosure of the dead tank circuit breaker thus further reducing the diameter and overall size of the bushing. Further, the transition shield maybe an integral part of the FRP tube.
Whited, et al. in the U.S. Pat. No. 5,153,383 disclosed a flexible sheet of material for mounting about a vertical or horizontal type porcelain insulator of electrical equipment to physically protect the insulator from falling objects during maintenance when the electrical equipment is de-energized. The sheet is formed of flexible PVC material and is foldable into a conical configuration and secured about the insulator by VELCRO strips attached to the sheet. Additional rectangularly-shaped sheets of similar material also are provided with VELCRO fasteners are formed into a cylindrical configuration and placed around the insulator and secured to the conically wrapped sheet to provide physical protector for taller porcelain insulators.
Eggleston in the U.S. Pat. No. 4,985,599 disclosed a bushing cap provided for mounting to a transformer bushing or to a lightening arrestor. The bushing cap when mounted to the bushing acts as an umbrella for the bushing to protect the same from airborne contaminates such as salt spray which can form a conductive surface on the bushing, while providing adequate bottom and side clearance from the bushing to ensure that a conductive surface between the cap and the bushing is avoided.
Thiel, et al, in the U.S. Pat. No. 4,760,216, disclosed a method for making an electrical bushing having an enlarged intermediate portion and reduced portions at opposite ends. The method is characterized by the steps of mounting an annular flange around one of the reduced portions to form a peripheral gutter adjacent to the one portion, mounting a cap on the other of the reduced portions to form a peripheral gutter adjacent to the other portion, placing a solid solder ring in each gutter, and with the open sides of the gutters upright, heating the solder rings to a molten temperature and cooling the molten solder to below its melting temperature in order to provide a joint between the flange and the one portion and the cap and the other portion.
Wood, et al. in the U.S. Pat. No. 4,670,625 disclosed a bushing arrangement for conveying an electrical conductor through the casing of high voltage electrical apparatus exposed to the weather comprises a rigid member of synthetic resin cast around, so as to be bonded in a gas tight manner to, the conductor, and incorporating an exterior portion protruding outside the casing, the member being enclosed, over at least the major part of its exterior portion, by a plurality of insulting axially-overlapping weather-resistant collars and a stress cone each having a creepage flange or shed, the member being secured to the casing by a flanged collar. The insulating member may be made of a relatively cheap non-weather-resistant epoxy resin and the collars of EPDM rubber.
Gamble in the U.S. Pat. No. 4,016,359 disclosed an insulating bushing assembly for use with electrical apparatus and adapted to be installed through an aperture in a wall thereof, characterized by an electrically insulating bushing extending through an aperture in the wall and sealed in place by an elastic gasket between the bushing and the wall on one side thereof, the bushing having an annular groove on the other side of the wall and a garter spring disposed in the groove and having a smaller portion thereof extending beyond the groove and in contact with the other side of the wall, whereby the elastic gasket is retained in fluid-tight compression between the bushing and the wall.
West, et al. in the U.S. Pat. No. 5,281,767 disclosed an improved, failure-resistant, low mechanical stress electrical connector bushing is provided which includes an elongated, central metallic conductor rod with an insulative epoxy body cast about the central rod. The rod is of substantially constant diameter throughout its length, and includes a male connection end and an opposed female connection end. The connection end presents an upset, integral, cold-forged, radially expanded terminal portion having a diameter greater than the constant diameter rod, and presents a butt end face. Stress analysis confirms that the bushing develops reduced, mechanical thermally induced stresses, as compared with a prior design.
Hamm in the U.S. Pat. No. 4,965,407 disclosed a bushing system for high voltage electrical equipment includes a weathershed with integral annular shirts molded around an electrical conductor. The weathershed has a cavity at its flange for mating with the opening in the housing for the high voltage electrical equipment. A dielectric gas in the housing communicates with the cavity in the weathershed. A flange and seal are provided at the flange of the weathershed for sealingly connecting the bushing system to the housing. A secondary ground may also be provided
Rupprecht, in the U.S. Pat. No. 4,847,450 disclosed a high voltage bushing formed from a plurality of insulating layers. Conductive layers are interleaved with the insulating layers, and are electrically insulated from each other. The ends of the conductive layers are overlapped by stress grading layers, to minimize the electrical stress on the bushing.
Mikulecky in the U.S. Pat. No. 4,818,967 disclosed a current limiting fuse for a transformer bushing including a glass tubing, an elongate ribbon spirally wound in the tubing, the ribbon including a number of tabs formed integral therewith to space the ribbon inwardly of the walls of the tubing, a terminal stud mounted on one end of the tubing and being electrically connected to the ribbon, a conductive rod connected to the other end of the ribbon and sealed in the other end of the tubing, a granular dielectric material completely filling the tubing, a self-sealing, self-bonding, rubber tape wrapped around the outside of the tubing and a glass reinforced tape wrapped around the outside of the rubber tape.
Brealey in the U.S. Pat. No. 4,477,692 disclosed a high-voltage terminal bushing comprising a tubular shell of electrical insulating material having a passageway extending between its ends and an electrical conductor within the passageway that has an external surface spaced from the internal surface of the passageway. The external surface of the conductor is covered with a thin coating of insulating material spaced from the internal surface of the passageway by a cylindrical air gap. The coating has sufficient dielectric strength to prevent any corona streamers from the conductor from impinging against said internal surface at impulse voltages up to the rated impulse insulation level of the bushing.
Koch, et al. in the U.S. Pat. No. 6,452,109 disclosed a bushing for a high electric voltage has a casing through which a central conductor is passed so that it is electrically insulated. Conducting liners spaced a distance apart are arranged concentrically around the central conductor, a combination which functions as a dielectric and is composed of one layer of film and one layer of a non-woven film being provided between the liners. The film and the nonwoven material are each made of plastic, so their water content is very low, and therefore the drying time in manufacturing of the bushing is shortened.
Therefore, the present invention recognizes that a need still exists for a durable, cost effective method for reducing the external creepage and incidents of arcing when using bushings rated at 45 W and 1.2/50 xcexcs without external flashover.
It is therefore one object of the present invention to increase the basic lightning impulse level of 1.2 kV class bushing from 30 kV to 45 kV.
It is another object of the present invention to decrease the external creepage and incidents of arcing when using bushings rated at 45 kV and 1.2/50 xcexcs without external flashover.
To exceed a minimum standard mechanical strength of 1000N for a cantilever load together with a reduction in size and weight with respect to other commercially available bushings withstanding a lightning impulse voltage of 30 kV.
The foregoing objects are achieved as is now described. The low-voltage porcelain bushing according to the present invention consists of a body and a shank. The body is constituted by a flange, a neck and a mouth. The body and the shank provide the main dielectric capabilities to the porcelain insulator and are designed to withstand 45 W standard lightning impulse voltage with a 1.2/50 xcexcs form, without being punctured or giving rise to a flaming arc. The body and the shank are also designed to operate under the maximum efficient tension generated between the metallic conductor and the mounting clamp, normally connected to earth.
The body is coated with glassy enamel in order to protect the porcelain material against contamination by external agents as well as against the absorption of water.
The geometry and dimensions of the body determine the withstanding level of the porcelain bushing to the lightning impulse voltage by establishing particular values for the creepage distance (shortest distance along the surface of an insulator between two conductive parts) as well as for the arcing distance (shortest distance in air external to the insulator between metallic parts which normally have the operating voltage between them).
The main feature of the bushing""s body of the present invention is the curvature radius of the neck. This curvature radius is such that allows the bushing to withstand lightning impulse voltages rated at 45 kV and 1.2/50 xcexcs form without external flashover or puncture. The curvature radius of the neck defines the creepage to arcing distance ratio. Extensive experimental testing carried out during the development of the present invention showed that the best performance of the bushing was achieved for curvature radii of the neck ranging from 5 to 7 mm. These curvature radii defined creepage to arcing distance ratios from 1.05 to 1.24, which correspond to one of the best electrical performance for the commercially available bushings.
An additional feature of the isolator body is the definition of a minimum curvature radius of 1.5 mm at the joints formed between the neck and both mouth and flange.
The above features, and advantages of the present invention will be best explained in the following detailed written description.